The present invention relates to an automatic gain control device for controlling, in a communication system or in an audio system, the gain of a variable gain amplifying circuit depending on the amplitude of an input signal so as to produce an output signal having a constant amplitude and to suppress a variation in the input signal.
Referring now to the drawing, a description will be given below to a conventional automatic gain control device with which a variation in an input signal can be suppressed and from which a constant output signal can be obtained.
FIG. 11 is a view showing the structure of the conventional automatic gain control device. In the drawing are shown: a variable gain amplifying circuit 1 for amplifying an input signal voltage depending on a gain controlled by a control voltage; a peak detecting circuit 2 for detecting and holding a peak value of an output voltage of the variable gain amplifying circuit 1; a rectifying circuit 21 for rectifying the input signal voltage; a holding circuit 22 for holding a peak value of the voltage rectified by the rectifying circuit 21; a direct-current amplifying circuit 3 for outputting a voltage proportional to a difference between the inputted voltage and a reference voltage; a low-pass filter 4 for removing a high-frequency component from the input voltage of the direct-current amplifying circuit 3 and outputting an effective voltage; a resistance-voltage dividing circuit 51 for generating an appropriate reference voltage from a power-source potential Vcc; a first resistor R1 for dividing the power-source potential Vcc; a second resistor R2 for dividing the power-source potential Vcc; a switching buffer circuit with a control terminal 61 for switching between the state in which the input signal passes through the variable gain amplifying circuit 1 and tile state in which the input signal does not pass through the variable gain amplifying circuit 1; an input terminal 61A to which the input signal is inputted directly; an input terminal 61B to which an output signal from the variable gain amplifying circuit 1 is inputted; an output terminal Y of the conventional automatic gain control device; a reference voltage Vref outputted from the resistance-voltage dividing circuit 51; a peak voltage Vx outputted from the peak detecting circuit 2; a control voltage Vc for controlling the variable gain amplifying circuit 1, which has been generated by the direct-current amplifying circuit 3 based on a difference between the peak voltage Vx and the reference voltage Vref and from which a ripple component has been removed by the low-pass filter 4; and the power source voltage Vcc for driving the device.
Below, the operation of the automatic gain control device thus constituted will be described with reference to the drawings.
FIGS. 12(a) to 12(c) are views each showing the AGC (Automatic Gain Control) characteristic of the conventional automatic gain control device. FIG. 12(a) is a view showing the relationship between the control voltage Vc and the gain of the conventional automatic gain control device, in which G0 designates a maximum gain and G9 designates a minimum gain.
FIG. 12(b) is a view showing the relationship between the peak voltage Vx and control voltage Vc of the direct-current amplifying circuit in which the reference voltage is held constant. In the drawing are shown: a value Vx1 of the peak voltage with which the gain becomes 0 dB when the reference voltage is Vr1 shown in FIG. 10(a); and a value Vc1 of the control voltage with which the gain of the variable gain amplifying circuit 1 becomes 0 dB when the peak voltage is Vx1.
FIG. 12(c) is a view showing the relationship between the voltage of the input signal of the conventional automatic gain control device and the voltage of the output signal thereof, each expressed in a logarithmic value. In the drawing are shown: a minimum value vin0 of the input signal voltage with which AGC becomes effective; a value vin1 of the input signal voltage with which the gain becomes 0 dB when the reference voltage is Vr1 shown in FIG. 10(a); a maximum value vin9 of the input signal voltage with which AGC becomes effective; the maximum gain G0; and the minimum gain G9. Since the gain decreases as the input signal voltage increases on the basis of vin1, while the gain increases as the input signal voltage decreases, the amplitude of the output signal voltage can be held constant.
As shown in FIG. 12(c), the gain becomes 0 dB when the control voltage is Vc1 shown in FIG. 12(b). When the reference voltage Vref inputted to the direct-current amplifying circuit 3 is varied from the standard value Vr1 to a higher value Vr3, the peak voltage shifts from the standard value Vx1 for outputting the control voltage Vc1 to a higher value, so that the input signal voltage also shifts from the standard value vin1 to a higher value vin3 with which the gain becomes 0 dB. Conversely, if the reference voltage Vref is varied to a value Vr2 shown in FIG. 10(a), the peak voltage shifts from the value Vx1 for outputting the control voltage Vc1 to a lower value, so that the gain becomes 0 dB when the input signal voltage is vin2, which is lower than the standard value vin1. Hence, the AGC characteristic of the automatic gain control device can be varied by varying the reference voltage Vref inputted to the direct-current amplifying circuit 3.
Next, a description will be given to a conventional automatic gain control device for voice with reference to the drawing.
FIG. 13 is a circuit diagram of the conventional automatic gain control device for voice. Here, a description will be given only to components newly added to the conventional automatic gain control device shown in FIG. 11. In FIG. 13 are shown: a clamping circuit 7 for holding constant an output voltage of a minute voltage detecting circuit when it exceeds a given value; a PNP transistor 72 which is brought into conduction when the output voltage of the minute voltage detecting circuit exceeds the given value; a constant-voltage power source 73 for determining the value of a clamping voltage; the minute voltage detecting circuit 8 for increasing the output voltage, as the input voltage is reduced, and rectifying it.
Below, the operation of the automatic gain control device thus constituted will be described with reference to the drawings.
FIGS. 14(a) and 14(b) are views showing the AGC characteristic of the conventional automatic gain control device for voice, of which FIG. 14(a) is a view showing the relationship between an input signal voltage and the control voltage Vc of the conventional automatic gain control device for voice and FIG. 14(b) is view showing the relationship between the input signal voltage and an output signal voltage of the conventional automatic gain control device for voice. In FIG. 14(a) are shown: an output voltage L1 of the rectifying circuit 21 shown in FIG. 13; an output voltage L2 of the minute voltage detecting circuit 8 shown in FIG. 13; an output voltage L3 of the clamping circuit 7 shown in FIG. 13; a value vin1 of the input signal voltage with which the gain becomes 0 dB when the reference voltage is Vr1 shown in FIG. 10(a); a value vin5 of the input signal voltage with which the output voltages of the minute voltage detecting circuit 8 and rectifying circuit 21 shown in FIG. 13 are equalized; a value vin6 of the input signal voltage with which the output voltages of the minute voltage detecting circuit 8 and clamping circuit 7 shown in FIG. 13 are equalized; and a value Vc1 of the control voltage when the peak voltage is Vx1, which corresponds to the values vin1 and vin6 of the input signal voltage with which the gain becomes 0 dB. In FIG. 14(b), G0 designates the maximum gain and G9 designates the minimum gain.
When the input signal voltage is smaller than vin1, the gain approaches the maximum value G0 and is eventually saturated. Even when the input signal voltage is extremely small, the gain is held at the maximum value without the minute voltage detecting circuit 8, resulting in an increase in noise. Since the minute voltage detecting circuit 8 generates a voltage higher than the voltage generated from the direct-current amplifying circuit 3 shown in FIG. 13 when the input signal voltage is vin5 or lower, the gain is reduced as shown in FIG. 14(b). On the other hand, the clamping circuit 7 is provided in order to forcibly apply the control voltage having the value of Vc1 to the variable gain amplifying circuit 3, since the gain becomes negative when the input signal voltage becomes lower than the vin6 only with the minute voltage detecting circuit 8.
With the above automatic gain control device for voice having the AGC characteristic, a large sound is attenuated, while a small sound has its S/N ratio improved, resulting in the reproduction of clear sounds.
However, the above conventional automatic gain control device has the following four problems: the first one is that a plurality of terminals are necessary for controlling the above conventional automatic gain control device, since the changing of the gain owing to the function of varying the reference voltage is separated from an AGC ON/OFF switching function; the second one is that the gain on receiving a minute input signal cannot be held at a constant value with respect to a variation in AGC characteristic when the reference voltage is variable; the third one is that it is impossible to freely set or store the AGC characteristic in accordance with an arbitrary level of the input signal; and the fourth one is that AGC ON/OFF switching cannot be performed at a high speed since the charging of the capacitance of the low-pass filter requires a considerably long period of time.